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CERAMICSINTERNATIONAL
Available online at www.sciencedirect.com
http://dx.doi.org0272-8842/& 20
nCorrespondinE-mail addre
2 (2016) 650–656
Ceramics International 4
www.elsevier.com/locate/ceramint
Microstructure and mechanical properties of B4C–TiB2 composites
preparedby reaction hot pressing using Ti3SiC2 as additive
Ping Hea,b, Shaoming Donga,n, Yanmei Kana, Xiangyu Zhanga,
Yusheng Dinga
aState Key Laboratory of High Performance Ceramics and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of
Sciences,1295 Dingxi Road, Shanghai 200050, China
bUniversity of Chinese Academy of Sciences, 19 Yuquan Road,
Beijing 100049, China
Received 30 July 2015; received in revised form 27 August 2015;
accepted 28 August 2015Available online 6 September 2015
Abstract
B4C–TiB2 composites were fabricated via reaction hot pressing at
2100 1C under a pressure of 25 MPa, using B4C and Ti3SiC2 powders
as rawmaterials. The phase transformations, microstructure and
mechanical properties were investigated by XRD, TG–DTA, SEM, TEM
and EDS. It isfound that the SiC and TiB2 particles are
homogenously dispersed in the B4C–TiB2 composites, where nano-sized
TiB2 particles are mainlylocated within the B4C matrix grains,
while the large-sized TiB2 particles at the matrix grains
boundaries. Due to the pinning effect of SiC andTiB2 particles on
B4C grain growth, the grain size of the composite is significantly
reduced, leading to a great improvement of the
mechanicalproperties. B4C–TiB2 composite prepared from B4C-10 wt%
Ti3SiC2 starting powder shows high flexural strength, fracture
toughness and micro-hardness of 592 MPa, 7.01 MPa m1/2 and 3163
kg/mm2, respectively. Crack deflection and crack bridging are most
likely the potentialtoughening mechanisms in the composites.
Furthermore, according to the XRD and TG–DTA analysis, the possible
reaction mechanisms leadingto the in-situ formation of TiB2 were
proposed.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights
reserved.
Keywords: Ti3SiC2; Reaction hot pressing; Phase transformation;
Microstructure and mechanical properties
1. Introduction
Boron carbide (B4C) ceramics has received wide attentionfor high
temperature structural applications owing to a numberof unique
properties, such as low density, high melting point,high hardness,
high elastic modulus and good abrasionresistance [1–3]. Despite
that, the full potential usage of thematerial is rather limited
because of its extremely poorsinterability (elevated sintering
temperature 42200 1C) andthe high susceptibility to brittle failure
( fracture toughness2–3 MPa m1/2).
Early studies showed that the addition of secondary phasesinto
B4C by forming multiphase composites, e.g. B4C/HfB2[4], B4C/CrB2
[5], B4C/TiB2 [6–9], B4C/SiC [10–11] andB4C/TiC [12], could
significantly improve the mechanical
/10.1016/j.ceramint.2015.08.16015 Elsevier Ltd and Techna Group
S.r.l. All rights reserved.
g author. Tel.: þ86 21 52414324; fax: þ86 21 52414903.ss:
[email protected] (S. Dong).
property as well as sinterability of the material. Among
them,B4C composite reinforced with TiB2 particles is
particularlyattractive due to its high melting point, high elastic
modulus,superior wear resistance and good chemical stability
[13–14].However, to achieve a good balance among different
aspectsof the mechanical properties of the material, it is still
difficult.For instance, hot-pressed B4C–TiB2 composite prepared by
Rushowed high strength and toughness of 506 MPa and9.4 MPa m1/2,
respectively, but the hardness was just only23.6 GPa [15]. In
comparison, hot-pressed B4C–TiB2 compo-site prepared by J.Vleugels
showed a combination of highstrength and high toughness of 867 MPa
and 29 GPa, while thefracture toughness was modest (4.5 MPa m1/2)
[16]. Similarphenomenon was also observed by Yamada. Reaction
hot-pressed B4C–TiB2 composite prepared by Yamada showedhigh
strength of 866 MPa, with a low toughness of about3.2 MPa m1/2
[17]. All these results indicate that more work
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P. He et al. / Ceramics International 42 (2016) 650–656 651
should be done to improve the full aspects of
mechanicalproperties of the materials.
On the basis of above, in this study, we explored thefeasibility
of preparation of the B4C–TiB2 composites viareaction hot pressing
using B4C and Ti3SiC2 powders as rawmaterials. We expected that the
addition of Ti3SiC2 reactedwith B4C and in situ generated TiB2, the
fine TiB2 grains weredispersed homogeneously in B4C matrix, forming
a kind ofcomplicated intragranular/intergranular microstructure
andenhancing the strength and hardness. In addition,
crackdeflection and crack bridging induced by TiB2 grains due
toresidual stress resulting from thermal expansion mismatchbetween
B4C and TiB2 improved the fracture toughness. Theresults show that
the composite obtained possesses a signifi-cant improvement of the
overall mechanical properties, and agood balance among strength,
toughness and hardness isachieved. The formation mechanism,
microstructure andmechanical properties of the composite are
discussed.
Fig. 2. XRD patterns of the as-received (a) B4C, (b) Ti3SiC2 and
(c) B4CþTi3SiC2 mixture powders.
2. Materials and methods
2.1. Materials
Commercially available Ti3SiC2 (Shanghai yuehuan newmaterial
technology co., Ltd., China, 98% purity) and B4Cpowders (Dalian
Jinma Boron Technology Group Co., Ltd.,China, 98.5% purity) were
used as starting materials. Fig. 1shows the SEM images of the as
received Ti3SiC2 and B4Cpowders. The Ti3SiC2 powder show aggregated
particles ofo5 μm in size, whereas the B4C powder is composed
ofirregular-shaped particles of o4 μm and relatively welldispersed.
X-ray diffraction (XRD) analysis shown in Fig. 2indicates that the
primary crystalline phases presenting in theTi3SiC2 and B4C powders
are Ti3SiC2 and B4C, with tinyamounts of B2O3 and TiC impurities,
respectively. Designatedamounts of B4C and Ti3SiC2 (0 wt%, 5 wt%,
10 wt%) powderswere weighed and mixed by ball milling in ethanol
for 24 hwith boron carbide grinding media, then dried in air
atmo-sphere. The resultant powder mixture was screened through
a60-mesh nylon sieve and stored for later use.
Fig. 1. Typical SEM images of the as-rece
2.2. Reaction hot-pressing
Reaction hot-pressing was carried out using a
hot-pressingapparatus in Ar atmosphere. The ball-milled B4Cþ
Ti3SiC2powder mixtures were loaded into a graphite die with
adiameter of 80 mm and a height of 150 mm, then heated to2100 1C at
a heating rate of 10 1C /min, and dwelled for60 min under a maximum
applied pressure of 25 MPa. Afterthat, the applied mechanical
pressure was removed and theelectric power was switched off to
allow the sample to cool toroom temperature.
2.3. Characterizations
The densities of the sintered samples were measured via
theArchimedes method using distilled water as the medium.Flexural
strength was measured through three-point-bendingtest on five
specimens (3� 4x40 mm3) with a span of 30 mmand crosshead speed of
0.5 mm/min by universal machine(Model 5566, Instron Company, U.K).
Fracture toughness wasevaluated by single edge notched beam test
with a span of16 mm and crosshead speed of 0.05 mm/min, five
sampleswere tested. The test bars, 2� 5� 30 mm3 in dimension,
werenotched by electromachining with a 0.2 mm diameter Mo line.
ived (a) Ti3SiC2 and (b) B4C powders.
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P. He et al. / Ceramics International 42 (2016) 650–656652
The notches were about 0.2 mm in width and 2.5 mm in depth.The
test bars were ground and polished with a series ofdiamond pastes
to a surface roughness of 0.5 μm, and theiredges were beveled
before testing. The microstructure wascharacterized by scanning
electron microscopes (SEM, JSM-6700F) and transmission electron
microscope (TEM, JEOL-2100F). The phase assemblages of the sintered
products wereanalyzed by X-ray diffraction (XRD) using CuKa
radiation at40 kV tube voltage and 450 mA current.
The reaction mechanisms during high temperature treatmentof the
B4Cþ10 wt% Ti3SiC2 mixed powder were investigatedas described
below. First, the thermal effects and weightchanges of the B4Cþ10
wt% Ti3SiC2 powder mixture duringheating was analyzed by
thermogravimetric (TG) and differ-ential thermal analysis (DTA)
techniques from room tempera-ture to 1500 1C with a heating rate of
10 1C/min in high purityAr (99.99%) atmosphere. Then, B4Cþ10 wt%
Ti3SiC2 mixedpowder were heat treated in Ar atmosphere from 900
to1600 1C for 1 h. The heat treated samples were characterizedwith
XRD, and the chemical reactions occurred in the systemwere
investigated.
3. Results and discussion
3.1. Reaction mechanisms of B4C and Ti3SiC2 mixture at
hightemperature
The TG–DTA curves of B4Cþ10 wt% Ti3SiC2 mixedpowder heat treated
from 100 to 1500 1C are shown inFig. 3. A broad endothermic peak
centered at 1295 1C wasobserved on the DTA curve, while two weight
losses occurringat the temperature ranges of 300–500 1C and
1100–1500 1Care observed on the TG curve. The low temperature
weightloss (0.41%) should correspond to the pyrolysis of
organiccontaminants introduced in the powder mixture during
ballmilling process, where a nylon pot was used. The weight lossat
high temperature is most likely associated with volatilizationor
the carbothermal reduction of B2O3 impurity in the samples,as will
be revealed later.
Fig. 3. TG–DTA curves of the as-received B4Cþ 10 wt% Ti3SiC2
mixturepowder.
The XRD patterns of B4Cþ10 wt% Ti3SiC2 mixed powderheat treated
from 900 to 1600 1C in Ar atmosphere for 1 h areshown in Fig. 4. It
can be seen that no obvious changes in theXRD pattern occur when
the heat treatment temperatures arebelow 1200 1C. The samples in
these cases show distinctivediffraction peaks of only B4C, Ti3SiC2,
TiC and B2O3, noadditional new phases are detected. This indicates
the absenceof noticeable reaction between Ti3SiC2 and B4C at
temperaturebelow 1200 1C. However, when the heat-treatment
temperatureincreased to be 1200 1C, radical changes in the XRD
patternoccur. The diffraction peaks of Ti3SiC2 and B2O3
disappear,meanwhile diffraction peaks of TiB2 new phase are
noticed,together with an obvious increase in the peak intensities
ofTiC. Such radical changes in the XRD pattern implies thecomplete
reaction between Ti3SiC2 and B4C to form TiB2 andTiC (reaction
(1)), as well as the removal of B2O3 from thesample through
chemical reaction with carbon (reaction (2)).
B4CþTi3SiC2-2TiB2þTiCþSiCþC (1)
2B2O3þ7C-B4Cþ6CO (2)
As the heat-treatment temperature further increased,
thediffraction peaks of TiC phase are reversely decreased. At
thesame time, the diffraction peaks of TiB2 become
continuouslystronger. At 1600 1C, no diffraction peaks of TiC can
bedetected, suggesting the complete conversion of TiC to
TiB2through reaction with B4C (reaction (3)).
B4Cþ2TiC-2TiB2þ3C (3)
On the basis of the above results, the overall reaction in
thesystem during heat-treatment of the B4Cþ10 wt% Ti3SiC2powder
mixture can be described as reaction (4), if the B2O3impurity in
the starting powder is ignored:
3B4Cþ2Ti3SiC2-6TiB2þ2SiCþ2C (4)
This reaction starts at 1200 1C and ended at 1600 1C withthe
in-situ formation of TiB2 and SiC. However, because thecontent of
SiC formed in the material is very small (2 wt%)and may be below
the detecting limitation of XRD technique,its presence in the
material is not reflected on the XRD patternsas found above.
3.2. Mechanical properties and Microstructure
Table. 1 lists the composition, density, flexural
strength,fracture toughness and hardness of the three ceramics
inves-tigated in this study. It can be seen that the composite
preparedwith 10 wt% Ti3SiC2 addition is nearly full dense with
arelative density of 99.6%, and optimal flexural strength,toughness
and hardness of 592 MPa, 7.01 MPa m1/2 and3163 kg/mm2 respectively.
Whereas monolithic B4C sinteredunder similar conditions achieved a
relative density of 98.6%together with relatively low flexural
strength, toughness and
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Fig. 4. XRD patterns of the as-received B4Cþ10 wt% Ti3SiC2
powder mixtures heat-treated at different temperatures for 1 h.
Table 1Mechanical properties of the B4C–TiB2 composite.
wt% of Ti3SiC2 Measured density g/cm3 Calculated density g/cm3
Relative density % flexural strength MPa fracture toughness
MPa.m1/2 Hv kg/mm2
0 2.4970.01 2.52 98.8 389728 4.4370.23 30837555 2.5370.01 2.58
98.1 533752 6.7370.26 314075610 2.6470.01 2.65 99.6 592710
7.0170.28 3163785
P. He et al. / Ceramics International 42 (2016) 650–656 653
hardness of 389 MPa, 4.43 MPa m1/2 and 3083 kg/mm2. Thepossible
explanation for the improved density can be inter-preted as
follows. The chemical reaction between B4C andTi3SiC2 will cause
deviations of B4C from its stoichiometry,and thereby increase the
concentration of structural vacanciesand lattice distortion in it
[18]. This activates the latticediffusion of boron and carbon
atoms, leading to enhancedmass transport and densification [18–19].
In addition, theapplication of high pressure for consolidation
results in highstress at the particle contact points. The stress
gradient at thecontact points can act as a driving force for mass
transport,which is beneficial for sintering [19].
SEM micrographs of the fracture surface of the B4Cceramics and
composites support the mechanical results shownin Table 1. As shown
in Fig. 5(a), the two composites exhibitsimilar XRD pattern, there
are only diffraction peaks of B4Cand TiB2, without any additional
phases. The correspondingback-scattered electron images of fracture
surface of the twocomposites are also shown in Fig. 5(c) and (d).
In the figure,the dark gray phases are B4C and the small bright
white phasesare TiB2. White TiB2 phases of fine sizes are
homogenouslydispersed in the dark gray B4C matrix, which mainly
resultsfrom the reaction between B4C and Ti3SiC2. Especially for
thecomposites with 10 wt% Ti3SiC2, the mean size of TiB2 grainsis
less than 1 μm and a large amount of sub-micrometer TiB2grains are
dispersed within the B4C grains, indicating theformation of
intragranular TiB2 structure in the composite. Inorder to
distinguish SiC phase, back-scattered electron imagesof surface of
the composites with 10 wt% Ti3SiC2 arepresented in Fig. 5(e). EDS
(Fig. 5(f)) shows that the gray
phases are SiC, and SiC phases are dispersed uniformly in
thedark gray B4C matrix, mean size of SiC grains is below 1 μm.As
for the smaller TiB2 grains, TEM were used for furtherobservation.
Fig. 6 shows representative TEM images ofintragranular and
intergranular TiB2 grains and the high-resolution. The black zones
located in B4C grains by EDSand electron diffraction analyses are
TiB2 grains with ahexagonal crystal structure. According to the
difference intheir location, TiB2 particles can be classified into
two types,i.e. intragranular and intergranular ones. The
intragranularTiB2 particles are confined within large boron carbide
grains,their sizes are in the range of tens to several
hundrednanometers in Fig. 6(a). As shown in Fig. 6(b), the
inter-granular TiB2 particles are located at the matrix
grainboundaries or triple junctions, and generally above
onemicrometer in size, much larger than their
intragranularcounterparts. It should be noted that the grain
boundariesbetween SiC, TiB2 and B4C grains are not as straight as
B4Cgrain boundaries. The irregularly curved grain
boundariesresulting from SiC and TiB2 grains indicate the pinning
effectof SiC and TiB2 grains on B4C grains. In addition, the
nano-particles or sub-micrometer particles located within B4C
grainsare prone to form sub-boundary structure in composites
underthe presence of internal stress, which can refine matrix
grainsshown in Fig. 5(d) and contribute to the improvement of
themechanical properties of the composites, the potential
hardnessdecrease caused by SiC and TiB2 formation in the composite
iswell compensated.The SEM micrographs of the crack propagating on
the
polished surface of B4C–TiB2 composite are shown in Fig. 7.
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TiB2
SiC
B4C
TiB2
B4C
TiB2
Fig. 5. (a) XRD, (b–d) SEM images of the fracture surfaces of
B4C–TiB2 composites prepared from B4CþTi3SiC2 mixtures containing
(b) 0 wt%, (c) 5 wt% and(d) 10 wt% of Ti3SiC2 and (e–f) SEM images
of surface and EDS of B4C–TiB2 composites prepared containing 10
wt% Ti3SiC2.
P. He et al. / Ceramics International 42 (2016) 650–656654
It can be seen that in situ formed TiB2 phase does not
changefracture mode of the monolithic B4C ceramic, which is
stilldominantly transgranular fracture. It can be also
concludedfrom transgranular fracture that the good bonding
strengthexists between B4C and TiB2 phase interface. As shown
inFig. 7, crack bridging and obvious crack deflection by
micron-sized TiB2 grains can be seen in the crack propagation. Due
tothermal expansion coefficient difference between B4C(4.5� 10�6/k)
and TiB2 (8.1� 10�6/k), residual thermalstress arises in the
composite [20–22]. The stress field imposesa strong impact on the
crack propagation behavior, makingcrack deflection and bridging by
the TiB2 secondary phase
particles. This may be effective barriers for crack
propagationand can consume much crack propagation energy. In this
way,the fracture toughness of the composite is improved.
4. Conclusions
In short, B4C–TiB2 composite of high flexural strength,hardness
and fracture can be prepared by the reaction of B4C andTi3SiC2
powder route with hot pressing. TiB2 particles are locatedboth
intragranularly and intergranularly in the B4C matrix,showing a
strong pinning effect on B4C grain growth. Thisresults in a
significant refinement of the microstructure of the
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TiB2
B4CTiB2
B4C
Fig. 6. TEM images of B4C–TiB2 composites showing (a) nano-size
TiB2 particles entrapped in B4C grains and (b) large-size TiB2
particles located at matrix grainboundaries or triple junctions;
(c) EDS and (d) corresponding electron diffraction and high
resolution image of a TiB2 particle.
Crack bridging
Crack deflection
TiB2
Fig. 7. Crack propagation behavior of the polished surface in
B4C–TiB2composite.
P. He et al. / Ceramics International 42 (2016) 650–656 655
composite and great improvement of its mechanical
properties.Apart from that, crack deflection and crack bridging by
micron-sized TiB2 grains may be also the main toughening
mechanisms.The composite fabricated from B4C-10 wt% Ti3SiC2
startingpowder shows the optimal mechanical properties, with
flexuralstrength, toughness and hardness of 592 MPa, 7.01 MPa m1/2
and
3163 kg/mm2, respectively. In comparison with the
B4C–TiB2composites reported before, a well balance in the
strength,toughness and hardness of the material is achieved.
Acknowledgments
The financial support from the “National High TechnologyResearch
and Development Program” (No: 2013AA030703),“National Natural
Science Foundation of China” (No:51272265) and “The Innovation
Project of The Chinese Academyof Sciences, Shanghai Institute of
Ceramics” (No: Y22ZC6160G)are greatly acknowledged.
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Microstructure and mechanical properties of B4C–TiB2 composites
prepared by reaction hot pressing using Ti3SiC2 as
additiveIntroductionMaterials and methodsMaterialsReaction
hot-pressingCharacterizations
Results and discussionReaction mechanisms of B4C and Ti3SiC2
mixture at high temperatureMechanical properties and
Microstructure
ConclusionsAcknowledgmentsReferences